SYNTHESIS AND CHARACTERIZATION OF ZINC ......The zinc tungstate oxides films prepared by advance controlled chemical spray pyrolysis deposition technique using double nozzle by spray

VOL. 14, NO. 4, FEBRUARY 2019 ISSN 1819-6608

ARPN Journal of Engineering and Applied Sciences ©2006-2019 Asian Research Publishing Network (ARPN). All rights reserved.

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SYNTHESIS AND CHARACTERIZATION OF ZINC TUNGSTATE OXIDES

FILMS BY ADVANCED CONTROLLED CHEMICAL SPRAY PYROLYSIS

DEPOSITION TECHNIQUE

Zena A. Salman, Alaa A. Abdul-Hamead and Farhad M. Othman Materials Engineering Department, University of Technology, Baghdad, Iraq

E-Mail: [email protected]

ABSTRACT

For the first time zinc tungstate semiconductor oxides films (ZnWO4) was successfully synthesized simply by

advanced controlled chemical spray pyrolysis deposition technique, via employed double nozzle instead of single nozzle

using tungstic acid and zinc chloride solutions at three different compositions and spray separately at same time on heated

silicone (n-type) substrate at 600 °C, followed by annealing treatment for one hour at 500 °C. The crystal structure,

microstructure and morphology properties of prepared films were studied by X-ray diffraction analysis (XRD), electron

scanning electron microscopy (SEM) and atomic force microscopy (AFM) respectively. According to characterization

techniques, a material of well-crystallized monoclinic phase ZnWO4 films with rod-type 1D microstructures close to

needle structure were obtained from using this advance technique, with thickness about 500 nm. Such these structures have

been recognized as one of the most efficient microstructures especially in gas sensor applications due to their large specific

surface area.

Keywords: ZnWO4, semiconductor oxides, thin film, advance spray pyrolysis method, microstructure characterization.

1. INTRODUCTION

There is a proceeding requirement for particularly

designed semiconductors that has prompted an enthusiasm

in ternary oxides. Ternary oxides give more noteworthy

adaptability to tune the chemical and physical

characterization of the materials by changing their

structure [1]. Pairing distinctive semiconductors permits

the vectorial relocation of electrons starting with one

semiconductor then onto the next, prompting an

increasingly productive electron/gap partition and more

noteworthy catalytic reactivity [2]. The control of

semiconductor composition, Morphology, and

microstructure are required for improving the

characteristic of different oxides [3].

In the current years, tungsten compounds have

been paid lots of attention because of their fantastic

physical and chemical characterization. Tungsten

compounds such as, tungsten oxides, carbides, nitrides,

sulfides, bronzes, tungstate, tungsten metal has very

wealthy chemistry materials, and they're all important

commercial materials. They can be utilized in catalysis,

electrical applications, photovoltaic cells, humidity and

gaseous detecting and different chromogenic fields,

scientific and dental applications [4].

Metal tungstate (MWO4) were solidified with the

wolframite kind of structure which can be depicted as

composed of hexagonal close-packed oxygen atoms with

specific octahedral locales filled by M2+

and W6+

cations

in an arranged way. Combination having a place with this

family was experienced as gas sensors or catalysts. The

metal tungstate family mixes, for example ZnWO4,

MnWO4, FeWO4, CoWO4, NiWO4, SnWO4 and CuWO4

have been mostly researched [5]. Specifically zinc

tungstate (ZnWO4) possessing a high application potential

in various fields, such as scintillator material,

photoluminescence, electronic and optical properties,

photovoltaic property, humidity sensor, hydrogen sensor,

ether sensor, photo-catalyst and high-power lithium-ion

batteries [6]. It was recognized as essential photo catalysts, optical fibers, gas detector and solid-state laser hosts [7].

In recent years, there has been growing interest in zinc

tungstate (ZnWO4) as a possible new material for sensor

material for detection gases [8]. At ambient conditions,

ZnWO4 crystallize with wolframite structure that has

monoclinic unit cell [9]. ZnWO4, is a wide-gap

semiconductor, with band gap energy close to 4 eV [10].

Numerous oxide blend can be custom fitted to

accomplish wanted surface to volume proportions and

morphologies as to achieve different gas detecting

efficiency. The addition of a second component may cause

a decline in the grain size, which likewise enhances gas

sensor reaction properties [11]. Blending of metal oxides

in a sensor layer has lately been investigated to enhance

sensor efficiency and thermal stability. The blended oxides

can possibly profit by the best detecting properties of their

unmixed oxides. The electronic structures of the oxides are

altered, bringing about change to both the mass and

surface characterization [12]. Numerous researchers have

investigated the possibilities of various morphologies of

metal oxide semiconductor (MOS) one dimensional

(1D)for example, wires, belts and needles micro and nano-

structured materials as field-producers because of their

low work capacities, high aspect proportions, high

mechanical dependable qualities and conductivities, and

wide applications in materials science, and especially in

gas detecting applications [7]. It is important that the

affectability of substance gas sensors is unequivocally

influenced by the particular surface of detecting materials.

A higher particular surface of a detecting material prompts

higher sensor affectability. Subsequently, numerous

systems have been received to build the particular surface

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of detecting films with fine structured, taking advantage of

the large specific surface of fine structured materials [13].

The properties of ZnWO4were observed to be firmly

linked with its morphology, crystallinity and particle size

distribution subsequently relies upon its way of

preparation [14]. ZnWO4 particles have been synthesized

by various routes such as polymerized complex method,

microwave assisted technique, hydrothermal, ligand-

assisted hydrothermal, template-free hydrothermal, solid-

state reaction, polyol-mediated synthesis, solid-state

metathetic approach, mechanochemical synthesis, sol-gel,

calcining co-precipitated precursor and combustion

method, electro-deposition and high direct voltage electro-

spinning process [6]. In spray pyrolysis method has been

carried out to board range of synthesis thin and thick

layers. Even multi-layered dense and porous films and

powders can be readily synthesis utilizing this adaptable

method [15]. These layers were utilized in different

equipment, for example, solar cells, sensors, and solid

oxide fuel cells [13].The properties of precipitate layer

relay on the conditions of fabrication [16].

Therefore, we report a technique that could

success-fully prepare ZnWO4 film by advanced controlled

chemical spray pyrolysis deposition technique, the suggest

process is easy, rapid, clean and actively efficient for

preparation of microcrystalline materials with controlled

size and shape and high density of surface area, which are

suitable for technological applications such as gas sensor

application.

2. MATERIALS AND EXPERIMENTAL WORKS

2.1 Materials used and preparation method

The zinc tungstate oxides films prepared by

advance controlled chemical spray pyrolysis deposition

technique using double nozzle by spray aqueous solutions

of tungstic acid (H2WO4) and zinc chloride (ZnCl2)

separately at same time with molarity (0.1 M) at three

different compositions are summarized in Table-1. The

material mass was determined according to equation

(W=MW*VL*M/1000), where Mw is molecular weight of

material (gm/mol), M is material molarity (mol/L), VL is

volume of distilled water (ml) and W is material mass

(gm) [17].

Table-1. Mixing percentages of salts.

Salts Samples mix %

S S1 S2 S3

H2WO4 100% 3 1 1

ZnCl2 0 1 1 3

Silicon wafers were used to deposit films which

was n-type, orientations ˂100˃ with resistivity of (0.65-

0.95 Ω-cm), and (625µm) thickness. The silicon wafers

cut to (1cm2) dimensions and emersion in diluent (1:10)

HF: H2O for (10 min) to remove native oxides layer after

that put in ultrasonic bath with ethanol Alcohol for 15 min

then washed by distilled water and finally dried by air

blowing and wiped with soft paper. Samples preparation

shown in Figure-1.

Figure-1. Silicon wafers preparation.

The whole spray system is homemade consists of

the following: heater and thermocouple (k-type), double

nozzle 1mm diameter with valve, electrical timer, air

compressor, electrical gas valve and connectors as shown

in Figure-2.

Figure-2. Spray system.

2.2 Thin film deposition procedure

First the salts were dissolved after knowing the

molecular weights in a certain volume of distilled water

20ml and placed on the magnetic stirrer for 20 min until

the solution get homogeneous and to ensure that the

material is completely dissolved. The solutions were then

put into the bottle of the two nozzles and then the substrate

was placed in the middle of the surface of the heater and

left to reaches the required temperature which was




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measured using a thermocouple, the substrates were fixed

on heater by Maxi aluminum adhesive conductive tape and

heated to reach the required temperature, the optimum

substrate temperature was found to be 600 °C, because the

reaction is not completed in the less temperature. After all

these steps were done, then solution was sprayed and

make sure the droplets of solution is falling in regular

manner on the substrate surface. The spraying time of the

solution was controlled by electrical timer and electrical

gas valve, the deposition time was (3sec) each (1-2 min)

thus the substrates no loss the thermal stability. After

finishing the deposition, the substrate left to cool to the

ambient temperature to avoid any thermal stresses which

may cause to broke or get distortions in the layer.

The other parameters like pressure spray rate and

spray distance technique are summarized in Table-2. After

the deposition, the prepared samples were annealed for

one hour at 500 ˚C and let the samples cooling inside furnace. This step is done for improving the quality and

crystallinity of the films according to author [18].

Table-2. Process parameters.

Process conditions Value

Pressure 7 bar

Air flow rate 8 cm3/sec

Spray distance 25 ±1 cm

Spray solution size 20 ml

Feeding rate 2.5 ml/min

Spatter number 20

Period between Spatter 1-2 min

3. MATERIALS CHARACTERIZATION

The crystal structure and phase identification of

the films after annealing were characterized by X-ray

diffraction (XRD) inspection with radiation CuKα (λ=1.5406 °A),The X-Ray diffraction investigations were

conducted by using SHIMADZU XRD-7000 MAXima,

the target is Cu beam with an angle from (10 to 60) with

40 KV & 30 mA.

The microstructures of the samples were

investigated by scanning electron microscopy (SEM) is

one of the most commonly used surface analysis

techniques in which a wide range of scales and feature can

be observed. Scanning electron microscope attached with

energy dispersive X-ray (EDX) examination was used to

reveal the microstructure and chemical composition of

samples

The surface roughness test was performed by

using the surface roughness tester (AFM) device supplied

with a sensor which moves linearly along the measured

length.

The thickness of prepared samples was determine by

utilizing the optical interferometer method that is depend

on interference of the light beam reflection from sample

surface and substrate bottom. Laser type He-Ne (632 nm)

is used and the thickness can be obtained by using the

formula below [19], and was calculated to be

approximately 500 nm.

T = ΔXX ∗ λ2 (1)

Where:

T = Thickness of the film in (nm).

X = Width of fringe (cm).

ΔX = Distance between two fringes (cm).

λ = Length of wave of laser light (nm).

4. RESULTS AND DISCUSSIONS

4.1 Crystal structure characteristics The X-ray diffraction pattern was used to identify

the phase present and their crystallite size is displayed

between 10 and 60 2θ, which is the area in which the most intense peaks of tungstate oxides are observed. Figure-3

shows the X-ray spectra of pure tungsten oxide film WO3

deposited onto silicone substrate at 300 °C. It is found that

film is crystalline and corresponding with monoclinic

crystal structure, the three predominant peaks correspond

to the direction (002), (020) and (200). The diffraction

peaks are agreed with data given in card (JCPDS No. 43-

1035) data, as already reported by [20-22]. The XRD

pattern shows broad peaks with small crystallinity due to

small quantity of spray solution, recently researchers

found that there is a relation between the quantity of

solution and the thickness of layer deposited, by increasing

the amount of solution the thickness of the layer increases,

and the peaks intensity increases also due to increasing

thickness of prepared layered [23]. Other researchers

found that the substrate temperature and annealing

temperature affected the phase of the WO3 films prepared

by spray pyrolysis, they reported that the as-deposited film

below 350 °C exhibits either amorphous or consisted of

very small crystallites, and at 400 °C exhibits crystalline

structure [24] and [25]. No characteristic peaks of other

compounds were observed. Table-3. Show results data of

XRD sample (S). The average crystallite size of WO3

sample is found in the range 2.4-3.7 nm was calculated by

the Scherrer equation (D=kλ/βcosθ) [26], where:

D = Average crystallite size.

K = Scherrer constant.

λ = X-ray radiation wavelength.

β= Peak width at half height.

θ = Peak position corresponds.




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Figure-3. Shows the x-ray spectra of pure tungsten

oxide film (S).

Table-3. Results data of XRD sample (S).

WO3

2θ (deg.) (hkl) Intensity (c/s)

23.12 002 100

23.59 020 97

24.39 200 99

120 19

The Figures (4-6) show the XRD patterns result

for (S1, S2 and S3) samples respectively. All of XRD

spectrums could be indicated to highly crystallized

monoclinic zinc tungsten oxides ZnWO4 structure oriented

(111) at intensity100%, that is match with JCPDS-PDF

card file No. (15-0774), these findings confirm the

formation of ZnWO4 corroborating to the results from

XRD analysis. An investigation of these data demonstrate

that the relating values are predictable with results detailed

by other authors [2] and [27]. The sharp diffraction

features suggest the crystalline nature of all the samples.

An increase in diffraction intensity indicates an increase in

the crystallinity, which can be attributed to increasing

substrate temperature and the annealing of the samples.

In Figure-4 show XRD pattern of sample (S1), the main

peaks were indexed corresponding to a orthorhombic

structure ofWO3 which is match with the JCPDS-PDF card

file No( 20-1324), oriented at (001), (200), (021), (201),

(220) and (400), its observed due to high content of

tungsten salt and increasing of deposition temperature than

sample (S) in Figure-3, while other peaks were indexed to

monoclinic ZnWO4 structure oriented at (011), (110),

(111), (200) and (022). The sharp peaks of both phases

that indicated the substrate temperature is suitable to

obtain highly crystallized monoclinic ZnWO4. The XRD

patterns of the composites corresponds ZnWO4–WO3 only

implying that there are no impurity peaks. Table-4. Show

results data of XRD sample (S1).

Figure-4. Shows the x-ray spectra of sample (S1).

Table-4. Results data of XRD sample (S1).

Compounds 2θ (deg.) (hkl) Intensity

(c/s)

ZnWO4

23.85 011 40

54.58 110 35

30.50 111 100

30.70 -111 90

36.45 002 35

48.75 022 14

WO3

23.09 001 100

24.08 200 95

33.35 021 35

33.64 201 25

34.03 220 50

49.33 400 20

In Figure-5 show XRD pattern of sample (S2), all

diffraction peaks can be readily indexed to a pure

monoclinic phase of ZnWO4 (JCPDS card No. 15-0774),

No additional peak of other phases has been found, which

indicates that the synthesized sample is single-phase, due

to equivalent ratios of solutions.Table-5. Show results data

of XRD sample (S2). In Figure-6 show XRD pattern of

sample (S3), the XRD spectra was indicated to

combination of ZnWO4as the major and ZnO as minor

phases. The major peaks were indexed corresponding to

monoclinic phase of ZnWO4 (JCPDS card No. 15-0774).

It observed broad peaks of pure ZnWO4in beginning and

sharp in ending that indicating to exist more than one peak

at one position (2θ). The minor peaks are indexed on basis of the crystallographic data of the known structure

hexagonal ZnO all the diffraction lines agree with reported

values and match with the JCPDS data (036-1451) due to

increasing zinc salts in this sample. No characteristic




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peaks of other compounds such as WO3were observed.

Table-6. shows results data of XRD sample (S3).




(c/s)

ZnWO4

23.85 011 40

54.58 110 35

30.50 111 100

30.70 -111 90

36.45 002 35

48.75 022 14

WO3

23.09 001 100

24.08 200 95

33.35 021 35

33.64 201 25

34.03 220 50

49.33 400 20




(c/s)

ZnWO4

23.85 011 40

54.58 110 35

30.50 111 100

30.70 -111 90

36.45 002 35

48.75 022 14

ZnO

23.09 001 100

24.08 200 95

33.35 021 35

33.64 201 25

34.03 220 50

49.33 400 20

The mean crystallite size of ZnWO4 was

calculated from X-ray line broadening analysis using the

Scherrer equation above. The mean crystallite size of

ZnWO4 was found to be approximately 16.51, 19.37 and

24.62 nm of S1, S2, and S3 samples respectively, and

found to be 17.05nm of WO3 in sample S1 and 18.3 nm

ZnO of samples S3.No characteristic peak of impurity was

detected on XRD patterns meaning that the materials

exhibits a high degree of purity.

4.2 Microstructural characteristics (SEM&EDX)

The morphology of the prepared samples was

studied by SEM, Figures (7-14) show a microstructure

photographs with EDX charts of annealed samples. It is

revealed that the prepared precipitate is well-crystalline

formed under the current synthesis condition, which

agrees well with the results of XRD. In Figure-7 show

fibrousWO3microstructurelike network, and have a




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uniform, filamentous (like chain) surface morphology and

variable length filaments distribute all over the surface. It

could be observed that intensity of the filamentous

structure is low due to small quantity of spray solution,

and that agree with XRD result [23]. The chemical

composition EDX result in Figure-8 of sample (S) after

heat treatment index to pure WO3 film oxide in close

agreement with the nominal compositions and did not

show any impurity elements.

Figure-7. SEM micrograph image of the sample (S).

Figure-8. EDX charts of the sample (S).

In figure-9 shows a representative SEM image of

as-annealed sample (S1) thin film.There is a low quantity

of rod-type 1D microstructures close to needle structure

with different length, is found in the range 3-4µm, and 0.5

µm width. This is consistent with what previous

researchers have found [27] and [7]. The SEM image also

reveals presence smaller aggregated of WO3 due to high

tungsten precursor concentration in this sample. The

chemical composition of sample (S2) thin film was

validated by EDX analysis. Figure-10 shows close

agreement with the nominal mixed compositions and did

not show any impurity elements. The EDX spectrum

showed the presence of only W, Zn and Oindicating a

successful preparation procedure, and that agree with SEM

and XRD results.

Figure-9. SEM micrograph image for the sample (S1).

Figure-10. EDX charts of the sample (S1).

In Figure-11 shows a representative SEM image

of as-annealed sample (S2) thin film. The microstructure

indicated to onlyZnWO4 phase, with high density of

aggregated uniform rod-type 1D microstructure comparing

with S1 sample, due to increase content of Zn salt. This

sample were composed of the whole micro-rods of

ZnWO4with the average length and diameter of 4µm

and1µm respectively. This microstructures possess high

surface area, thus by using this advanced technique The

chemical composition EDX result for the composition of

sample (S2) after heat treatment in figure-12 index to pure

ZnWO4 film oxide. Note only Zn, W and O were detected,

in close agreement with the nominal compositions and did

not show any impurity elements, and that agree with XRD

result.




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In Figure-13 shows a representative SEM image

of as-annealed sample (S3) thin film. It found by

increasing Zn salt content, ZnO oxide appear as major

phase with ZnWO4 oxide as minor phase. The high-

magnification SEM images show that microstructure

indicated to of rod-type 1D ZnWO4 oxideless dense

comparing to sample (S2). The average length of

ZnWO4micro-rode became fine and shorter close to be 2

µm and with width approximately 0.5µm. The chemical

composition EDX result for the composition of sample

(S3) after heat treatment in figure-12 index to composites

correspond ZnWO4–ZnO. Note also that only Zn, W and

O were detected, in close agreement with the nominal

compositions and did not show any impurity elements, and

that agree with XRD result



4.3 Morphological characteristics (AFM) The morphology of surface of samples was

analyzed by using atomic force microscope, and also used

to determine average diameter of particles of films and its

distribution. This surface characteristic is important for

applications such as gas sensors and catalysts. Figures (15-

18) shows the typical two and three dimensional AFM

image with granularity accumulation distribution chart of

samples. AFM images show that the all films are well

faceted crystallites, uniformly packed and were small

grain; this means that the prepared films are well

deposited.Table-7 show the variation of root mean square,

average roughness and average diameter of annealed

samples.

It's observed that the average roughness and root

mean square of pure WO3 film sample (S) S1 found to

be1.19 nm and 1.45 nm respectively, and it is increased

with decreasing the tungsten salt, that mean by addition of




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zinc salt the average roughness and root mean square

increase, and the highest value at sample (S2) 6.67 nm and

7.7 nm respectively, when the proportion of solutions is

equal, and became less in sample (S3) when the proportion

of zinc salt is reduced.

Granularity accumulation distribution charts of

all sample show that the particles on the surface of the film

have different diameter values with narrow range and

displays a statistical distribution for the particle diameter

over the surface and it acceptable value for gas sensor

application as reported by researcher [28].

Figure-15. AFM image (a) 3D, (b) 2D and (c) Granularity

accumulation distribution chart of the sample (S).


accumulation distribution chart of sample (S1).




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Table-7. Variation of root mean square, average

roughness and average diameter of samples.

Samples Roughness

average(nm)

Root mean

square(nm)

Diameter

average(nm)

S 1.19 1.45 54.89

S1 3.96 4.67 78.19

S2 6.67 7.7 70.75

S3 4.62 5.51 84.79

5. CONCLUSIONS a) It could conclude that spray pyrolysis technique using

double nozzles and the substrate temperature are

suitable to obtain crystallized monoclinic zinc

tungsten film ZnWO4.

b) Spray pyrolysis method by employed double nozzles

exhibits rod-type 1D microstructures close to needle

structure to ZnWO4microstructure and this type is

most efficient microstructures especially in gas sensor

applications due to large specific surface area.

c) From XRD and SEM results, it can be concluded that

the content of zinc salt influenced the phase and

morphology of ZnWO4film synthesized advanced

spray pyrolysis technique. Increase the zinc salt

content has altered the shape topography and

microstructure of the prepared samples; this is




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reflected in the changing ratio of the phases formed

with various percentages of zinc salt.

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SYNTHESIS AND CHARACTERIZATION OF ZINC ......The zinc tungstate oxides films prepared by advance controlled chemical spray pyrolysis deposition technique using double nozzle by spray